Recently, the following is strongly demanded of, e.g., a hydraulic reciprocating sealing structure: (i) to allow contributions to the reduced size of devices and increased pressure; and (ii) to reduce the number of parts by forming a ring-shaped groove into which a seal is inserted.
While in order to meet the demand (i), a sealing structure requires size reduction of a seal itself, an increase in the seal rigidity, and complication of the seal shape, these requirements result in a significant decrease in workability (ease of seal insertion) in inserting a seal into a ring-shaped seal groove (recess), and thus, the sealing structure cannot meet the demand (ii).
Furthermore, in order to meet the demand (i), a double sealing structure in which two seal grooves (recesses) are formed, and which includes a seal inserted into each of the seal grooves is employed as a sealing structure which can be used at high pressure. For example, in the invention described in PATENT DOCUMENT 1, a double sealing structure has been proposed in which a U-shaped first seal 46 and a U-shaped second seal 47 both made of rubber are inserted into two seal grooves (recesses) 40 and 41, respectively, as illustrated in FIG. 13, and a backup ring 42 made of synthetic resin is fitted to the back surface 47a of the second seal 47.
The two ring-shaped seal grooves 40 and 41 are formed directly in a cylinder head 44 through which a piston rod 43 is reciprocatably inserted. The first seal 46 and the second seal 47 are made of elastic rubber to ensure easy insertion of the seals, and the cross-sectional area of the backup ring 42 is set to an area as small as possible, thereby allowing a thin receiver piece (deformation prevention piece) 42A to extend from an end portion of the backup ring 42 located near the inside diameter thereof toward a fluid storage chamber 45.